The Tg (Glass transition temperature) is the temperature at which the PCB base material stops acting like a rigid sheet and becomes rubbery. This is an important characteristic of the PCB fabrication process.

Standard Tg is between 130 – 140℃, 
Middle Tg is between 150-160℃,
High Tg is anything above 170℃

The PCB is more difficult to manufacture as the Tg increases so the cost of your circuit board increases proportionally. Consider a higher Tg PCB if your custom electronics design must have unusually high resistance to heat, moisture, chemical, delamination, and other harsh characteristics. Standard Tg is fine for most applications.